Most electronic systems include multiple digital integrated circuits (ICs), such as PLDS, ASICs, memory devices, and processors, that are mounted on printed circuit boards (PCBs). Each PCB includes a pattern of printed metal lines (e.g., copper tracks) formed on a board of insulating material. The ICs are typically soldered to the copper tracks at specific locations on the PCB, and the copper tracks provide signal paths for transmitting binary signals between the ICs of the electronic system.
Before ICs are soldered to a PCB to form an electronic system, the ICs are typically tested to verify that all of the ICs properly communicate with each other. In particular, the ICs are tested to verify that the binary output signals (logic high and logic low) from a first IC are properly interpreted at the input terminals of a second IC. This testing involves measuring the low-level input voltage (Vil) and high-level input voltage (Vih) of each IC of the system. Vil and Vih are direct current (DC) characteristics of all digital ICs. The Vil of an IC represents the maximum allowable input voltage that will be interpreted as a logic low (e.g. 0) by the IC. The Vih of an IC represents the minimum allowable input voltage that will be interpreted as a logic high (e.g., 1) by the IC. For example, a particular IC may interpret all input signals below 0.8 Volts as having logic low values, and all input signals above 2.0 Volts as having logic high values. Accurate measurement of Vil and Vih are important for predicting whether various ICs in the same electronic system can understand each other correctly.
FIG. 1 is a diagram showing a conventional system for measuring the Vil and Vih of an IC. The system requires a human operator 110 to manually control a power supply 120 using manual controls 122 and a display 125 such that power supply 120 transmits a desired test voltage from output terminal 127 to the input terminal 133 of an IC device-under-test (DUT) 130. Input terminal 133 of DUT 130 is connected to an output terminal 138 (i.e., such that the logic input signal at input terminal 133 produces a corresponding logic output signal at output terminal 138). Output terminal 138 of DUT 130 is probed by an oscilloscope 140 such that the output signal from DUT 130, which is generated in response to the applied voltage from power supply 120, is transmitted to an input terminal 142 of oscilloscope 140. Oscilloscope 140 includes a screen 145 from which human operator 110 visually observes a graphical representation of the output signal from DUT 130. Test data related to the Vil and Vih testing of DUT 130 is then manually entered by human operator 110 into a computer 150 using an input device (e.g., keyboard) 152 and a display 155.
FIG. 2 is a flow diagram showing the conventional method of measuring the Vil of DUT 130 (Vih is measured in a similar manner). For Vil measurements, an initial test voltage is set to VCC (e.g., 3.3 Volts) (Step 210). After connecting DUT 130 between power supply 120 and oscilloscope 140, as shown in FIG. 1, human operator 110 manually adjusts power supply 120 to apply the initial test voltage to DUT 130 (Step 220). The initial test voltage is significantly higher than the expected Vil for DUT 130 to assure that DUT 130 generates a logic high output signal (e.g., VCC). Human operator 110 then verifies that the output signal from DUT 130 is high by observing the graphical representation on screen 145 of oscilloscope 140 (Step 230). When the output signal from DUT 130 is high ("N" in Step 240), human operator 110 mentally calculates an incrementally decreased applied voltage (Step 250). This incremental voltage is calculated by subtracting a resolution value from the previously used applied test voltage. Human operator 110 selects the resolution value (i.e., degree of accuracy). After mentally calculating the adjusted applied voltage, human operator 110 manually adjusts power supply 120 to generate the adjusted applied voltage (Step 220) and observes the output signal from DUT 130 on screen 145 of oscilloscope 140 (Step 230). The process of incrementally decreasing the applied voltage and observing the output signal is repeated until a logic low output signal (e.g., ground or 0 Volts) is observed on screen 145 ("Y" in Step 240). This change in the output signal indicates that the actual Vil for DUT 130 is within the resolution range of the most recent applied voltage. This final applied voltage is then assigned as Vil for DUT 130 (Step 260), and human operator 110 manually enters this value into computer 150 (Step 270).
In practice, several problems are created by the conventional method of measuring vil and Vih. As indicated in FIG. 1, human operator 110 is used as a feedback control and must divide his attention between power supply 120, oscilloscope 140, and computer 150. If the applied voltage is changed too fast, operator 110 may miss the actual Vil/Vih voltage level. Further, because human operator 110 must manually enter the Vil/Vih values into computer 150, a rapid measurement pace may result in translation errors. Conversely, if the voltage is changed too slowly, the repetitious and meticulous measurement process becomes tedious for human operator 110, who may then make mental errors. This problem becomes even more significant when the measurement process is used to measure Vil/Vih for multiple terminals on multiple DUTs under varying conditions of VCC and temperature. Further, the significant amount of operator time to perform these measurements can significantly increase the overall cost of an electronic system incorporating DUT 130.
Other problems arise that are attributed to the use of oscilloscope 140 (see FIG. 1). If the ground level of power supply 120 is not exactly the same as the ground level of oscilloscope 140, then measurements read from screen 145 of oscilloscope 140 can be inaccurate by the difference between the two ground levels. For this reason, it is difficult to maintain a consistent and high resolution for all Vil and Vih measurements over all combinations of DUTs and terminals of an electronic system using the conventional method. This problem arises even when very expensive state-of-the-art instruments are used.
What is needed is a system and a method for measuring Vil and Vih that overcome the problems described above that are associated with the conventional system and method.